Mi perro es tan fiel como mi mejor amigo.

Yes. Tan … como with an adjective means “as … as”.

• tan fiel como = as faithful/loyal as
• Structure: tan + adjective + como + comparison term

So:

• Mi perro es tan fiel como mi mejor amigo.
  = My dog is as faithful as my best friend.

It expresses equality in the quality described by the adjective (fiel).

In Spanish:

• tan is used with adjectives and adverbs
  
  • tan fiel, tan alto, tan rápido
• tanto / tanta / tantos / tantas is used with nouns
  
  • tanto dinero (so much money)
  • tantos amigos (so many friends)

Because fiel is an adjective, you must say tan fiel, not tanto fiel.

Compare:

• Mi perro es tan fiel como mi mejor amigo.
  My dog is as faithful as my best friend.
• Tengo tantos amigos como tú.
  I have as many friends as you.

No, here you need es because fiel is seen as a characteristic / trait, not a temporary state.

• ser is used for:
  • essential characteristics: es fiel, es alto, es simpático
• estar is used for:
  • temporary states or conditions: está cansado, está enfermo

Loyalty is considered a more permanent trait, so you say:

• Mi perro es fiel. (He is loyal, as a characteristic.)

Mi perro está fiel would sound wrong or at least very strange in standard Spanish.

Fiel is one of the adjectives that does not change for gender:

• masculine singular: un perro fiel
• feminine singular: una perra fiel

For the plural, it adds -es:

• masculine plural: unos perros fieles
• feminine plural: unas perras fieles

So in your sentence:

• Mi perro es tan fiel como mi mejor amigo.
  perro = masculine singular → fiel (not fielo or fiela)

The structure tan + adjective + como is fixed in Spanish. You can’t move tan or como around.

✅ Correct:

• Mi perro es tan fiel como mi mejor amigo.

❌ Incorrect:

• Mi perro es fiel tan como…
• Mi perro es tan como fiel…

So always use: tan + adjective/adverb + como + noun/pronoun.

With mejor (better/best), the normal, natural position is before the noun:

• mi mejor amigo = my best friend
• mi mejor trabajo = my best job

You can sometimes see amigo mejor in special literary or poetic contexts, but in everyday Spanish from Spain, the standard is:

• mi mejor amigo, not mi amigo mejor.

So the sentence uses the usual, idiomatic order.

Spanish uses different words for different kinds of comparison:

• Equality (as … as):
  • tan fiel como = as faithful as
• Inequality (more/less … than):
  • más fiel que = more faithful than
  • menos fiel que = less faithful than

So:

• Mi perro es tan fiel como mi mejor amigo.
  My dog is as faithful as my best friend. (equality)

If you wanted to say your dog is more faithful, you’d say:

• Mi perro es más fiel que mi mejor amigo.

You want to express possession, so you use the unstressed possessive adjective:

• mi perro = my dog
• tu perro = your dog
• su perro = his/her/their/your (formal) dog

You could say el perro mío, but:

• mi perro is more common and more neutral.
• el perro mío sounds more emphatic or stylistic, like “that dog of mine”.

So for a normal sentence, mi perro is exactly what you should use.

The personal a is used mainly with direct objects that are people (or personified animals):

• Veo a mi amigo. (I see my friend.)
• Quiero a mi perro. (I love my dog.)

In your sentence, mi mejor amigo is not a direct object; it is simply the second term in a comparison with como:

• tan fiel como mi mejor amigo

Because it’s not a direct object, you do not use the personal a here. So:

• ✅ tan fiel como mi mejor amigo
• ❌ tan fiel como a mi mejor amigo

Yes, a very common alternative in Spain is:

• Mi perro es igual de fiel que mi mejor amigo.

This also means “My dog is just as faithful as my best friend.”

Patterns:

• tan + adjective + como
  
  • tan fiel como
• igual de + adjective + que
  
  • igual de fiel que

Both are natural and correct; tan … como is slightly shorter and very frequent, but igual de … que is also very common.